#include <cstdio>

 #include <cstring>

 #include <map>

 #include <cmath>

 using namespace std;

 #define LL __int64

 LL p[],o[];

 LL x,y;

 LL ext_eulid(LL a,LL b)

 {

     LL t,d;

     if(b == )

     {

         x = ;

         y = ;

         return a;

     }

     d = ext_eulid(b,a%b);

     t = x;

     x = y;

     y = t - (a/b)*y;

     return d;

 }

 LL gcd(LL a,LL b)

 {

     return b == ?a:gcd(b,a%b);

 }

 int main()

 {

     int i,n,z;

     LL p1,o1,d,c,mod;

     while(scanf("%d",&n)!=EOF)

     {

         for(i = ;i < n;i ++)

         {

             scanf("%I64d%I64d",&p[i],&o[i]);

         }

         p1 = p[];

         o1 = o[];

         z = ;

         for(i = ;i < n&&z;i ++)

         {

             d = ext_eulid(p1,p[i]);

             c = o[i] - o1;

             if(c%d) z = ;

             mod = p[i]/d;

             x = ((c/d*x)%mod+mod)%mod;

             o1 = p1*x + o1;

             p1 = p1*mod;

         }

         if(!z)

         printf("-1\n");

         else

         printf("%I64d\n",o1);

     }

     return ;

 }